Effect of the particle size on the thermal stability of nanostructured aluminum powder: dislocation density and second-phase particles controlling the grain growth

A nanostructured aluminum powder was obtained using cryogenic mechanical milling. The powder produced after 25 h of milling showed a broad particle size distribution, ranging between a few microns up to about 150 μm. Five different granulometric classes were selected and for each of these, structural and microstructural features, as well as the thermal stability were investigated using ex situ X-ray diffraction and transmission electron microscopy. There is a direct correlation between particle size and crystallite size. Grain growth tendency was found strongly dependent to the initial grain size with noticeable changes on the thermal stability for the five granulometric classes considered. Particularly the quantity of nitrogen content measured (after degassing) in each of the five granulometric classes increases with decreasing the particles size. This might justify why the calculated drag stress exerted by segregated impurities, second-phase particles and pores is effectively higher for small particles. This approach can give a methodology to modulate the microstructure of bulk nanostructured/ultrafine metals just selecting different combination in terms of particle size.